The present invention relates to a control system for an internal combustion spark ignition engine which permits the engine to be run on less than all of its cylinders in order to achieve substantial economies in engine operation. Such a capability is sometimes referred to as "split-engine" operation through which it is possible to operate on one-half of the cylinders of an engine having an even number of cylinders. For example, with the present invention it is possible to operate on four cylinders of an eight-cylinder engine.
Engine operation is more economical if each cylinder of the engine is run under relatively high loads. However, under a large percentage of vehicle operating conditions the engine is operating under relatively light loads resulting in uneconomical fuel consumption. Accordingly, it is desirable to operate the engine on half of the cylinders during normal or light load operation with the remaining cylinders being brought into operation only after the load on the engine exceeds a given value. In this way it is possible to increase the load on each of the active cylinders and in that way achieve greater overall operating economies for the engine.
More specifically, to operate an engine at part load it has to be throttled and thus produces a manifold vacuum the production of which wastes a significant portion of the engine power. This is referred to as "throttling losses". On the other hand, operating an engine on one-half of its cylinders requires less throttling thereby reducing the vacuum produced and, in turn, greatly reducing "throttling losses". Also, during half-cylinders-operation combustion takes place in only the "active" cylinders as a result of which considerably less heat radiating surfaces are dissipating the combustion energy.